1. Field of the Invention
The present invention relates to a brake controlling apparatus for an electric vehicle, and more particularly to an apparatus for reducing vehicle speed utilizing regenerative braking of an electric motor and a friction braking force of a friction brake.
2. Description of the Related Art
To meet the need for low emission and low noise vehicles, various types of electric vehicles have been proposed, some of which have reached practical application. In the deceleration of electric vehicles, when acceleration is off or braking is on, the use of regenerative braking of an electric motor is widely known. Through regenerative braking, a regenerative torque is generated by the electric motor in a direction opposite to the driving operation and a braking force similar to that from engine braking in a vehicle equipped with an internal combustion engine is obtained. A driver can adjust the vehicle speed, such as when traveling down a slope, by releasing the accelerator, much in the same manner as using an engine brake. During regenerative braking, the electric motor operates as a generator. The generated electric power then charges a battery so as to improve energy efficiency.
Japanese Patent Laid-Open Publication No. Hei 5-161210 describes an electric vehicle using a combination of a regenerative braking force from an electric motor and a friction braking force from a friction brake. Here, friction brake refers to a mechanical brake that causes a vehicle to decelerate by a mechanically based frictional force. Wheel brakes are common friction brakes. In the apparatus of the above-mentioned publication, when the required braking force based on an amount of brake operation is small, the required braking force is attained using only the regenerative braking force. When the required braking force exceeds a maximum value of the regenerative braking force, the required braking force is attained using a combination of the regenerative braking force and the friction braking force.
The most popular type of conventional electric vehicle has had no transmission and has therefore had a fixed gear ratio between the motor and wheels because electric motors can generate sufficient driving torque across a wide rotational speed range and are quiet even at high rotational speeds. In contrast, vehicles equipped with an internal combustion engine have been heretofore provided with a transmission to ensure a sufficient torque at low vehicle speeds. However, in electric vehicles without a transmission, the electric motor is frequently used at high rotational speeds, thereby increasing the load on auxiliary machinery related to the electric motor. Consequently, for efficient use of the electric motor, the disposition of a transmission between the electric motor and wheels has been proposed. The electric vehicle described in the above-mentioned Japanese Patent Laid-Open Publication No. Hei 5-161210 is also equipped with a transmission.
In an electric vehicle equipped with a transmission, it is also preferable to improve energy efficiency by effectively using regenerative braking. For example, when a driver depresses a brake pedal, a regenerative braking force (or a combination regenerative braking force and friction braking force) are generated. When the driver changes a shift range from this state by operating a shift lever, downshifting is performed corresponding to the range change. A gear ratio changes due to the downshifting so that a larger regenerative braking force is furnished to the vehicle.
However, when an attempt is made to perform this sort of braking control, the following problem arises. Generally, it is necessary to set the torque of the electric motor to approximately zero during gear shifting control in which the transmission switches a stage of gear. Therefore, in a case where the driver performs a lever operation to downshift during braking, the regenerative torque temporarily becomes zero while downshifting so that deceleration of the vehicle decreases. As the downshifting completes, a deceleration larger than that prior to the downshifting suddenly occurs. This sort of fluctuation in deceleration causes the driver to experience a poor ride. Under these circumstances, effective regenerative braking extending over a plurality of gear stages, namely, regenerative braking before and after gear shifting has not heretofore been known.